The invention relates to thermoplastic flexible films having a heat sealable layer, and especially relates to multilayer films suitable for packaging fresh or frozen foods such as meat, poultry or cheese.
Food manufacturers and wholesalers utilize flexible thermoplastic food packaging films to provide economical, sanitary containers which help preserve the freshness and wholesomeness of their food products. These films are often sold to food processors in bag form. For example, a single or multilayer thermoplastic film may be made into bags by a food packaging manufacturer using film stock comprising a tubular film or one or more flat sheets or webs of film by well known processes involving e.g. cutting, folding and/or sealing the film to form bags which may then be shipped to processors for use in food packaging operations. These films and bags may be printed with customer logos, product data or other information and may also be uniaxially or biaxially oriented, heat shrinkable, or irradiated, or may contain film layers which are abuse resistant or puncture resistant, or which are crosslinked or which enhance or retard or prevent transmission of light, gases, or liquids therethrough. Frequently, multilayer films having one or more barrier layers to oxygen and/or moisture such as saran (a polyvinylidene chloride copolymer), a modified saran e.g. containing methyl acrylate polymer units, ethylene vinyl alcohol copolymer, nylon, or acrylonitrile may be used with a heat sealing layer such as a copolymer of ethylene and vinyl acetate (EVA) to produce bags for packaging fresh red meat. Such bags help preserve the meat in its original condition by preventing or reducing moisture loss and chemical changes in the meat structure due to oxidation reactions. A typical bag produced from a tubular film stock will have one or two sides which have been heat sealed by the bag manufacturer in the bag forming process. Such bags will have one open side to allow the food processor to insert the ham, turkey, chicken, cheese, primal or subprimal meat cuts, ground beef, fruits, vegetables, bread or other food products into the bag. The food processor then makes the final seal thereby enclosing the bag. This final seal may follow gas evacuation of the bag by vacuum means or replacement of the gaseous environment within the bag by a particular gas or mixture of gases which may be inert or reactive with the enclosed product to provide some advantage such as to assist product preservation. This final seal is frequently a heat seal similar to the initial seals produced by the bag manufacturer although the actual heat sealing equipment may vary.
Thus, commercially available bags are made by transversely sealing a tubular stock of either monolayer or multilayer film and cutting off the tube portion containing the sealed end, or by making two spaced apart transverse seals on a tubular stock and cutting open the side of the tube, or by superimposing flat sheets of film and sealing on three sides, or by folding flat sheets and sealing two sides.
Generally heat sealing of thermoplastic film is accomplished by applying sufficient heat and pressure to adjacent film layer surfaces for a sufficient time to cause a fusion bond between the layers.
A common type of seal used in manufacturing bags is known to those skilled in the art as a hot bar seal. In making a hot bar seal, adjacent thermoplastic layers are held together by opposing bars of which at least one is heated to cause the adjacent thermoplastic layers to fusion bond by application of heat and pressure across the area to be sealed. For example, bags may be manufactured from a tube stock by making one hot bar seal transverse to the tube. This seal may also be referred to as a bottom seal. Once the bottom seal is applied, the tube stock may be transversely cut to form the mouth of the bag.
The strength of seals of heat shrinkable bags may be measured by determining the time for a seal to fail when under certain conditions the seal is immersed in hot water at 95.degree. C. i.e., the hot water seal strength ("HWSS") may be measured by a test such as that described as the "restrained shrinkage-seal strength test" in Funderburk et al U.S. Pat. No. 3,900,635 which patent is hereby incorporated by reference.
Once a food product such as meat or poultry is inserted into the bag, the package is typically evacuated and the bag mouth sealed. At one time, the standard method for sealing a bag was to fasten a clip around the mouth of the bag. More recently, heat sealing techniques have been employed to seal the bag. For example, a bag mouth may be hot bar sealed or it may be sealed by another common type of heat seal known as an impulse seal. An impulse seal is made by application of heat and pressure using opposing bars similar to the hot bar seal except that at least one of these bars has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name "impulse") to cause the adjacent film layers to fusion bond. Following the impulse of heat the bars are cooled (e.g. by circulating coolant) while continuing to hold the bag inner surfaces together to achieve adequate sealing strength.
Generally, impulse seals may be made faster than hot bar seals because of the quick cool down of the impulse ribbon following the heat impulse. Impulse seals are also generally narrower than hot bar seals which lead to an improved package appearance, but narrower seals also leave less margin for error in the production of continuous sealed edges. Since typically less area is bonded in an impulse seal relative to a hot bar seal, the performance of the sealing layer of the thermoplastic film is more critical.
One problem which occurs during impulse heat sealing of known films is that the film in the seal area often becomes extruded during sealing. This results in thinning of the film in the seal area and therefore reduces the strength of the film at the seal or in extreme situations, allows the thinned film to be too easily severed or pulled apart. Those skilled in the art refer to severely extruded seals as "burn through" seals. Thus, a "burn through" seal does not have adequate strength or integrity to seal in or protect the packaged product. One attempt to solve this "burn through" problem is to irradiate the film prior to manufacture of the bag.
Irradiation of a multilayer film causes the various irradiated layers in the film to crosslink. Under controlled conditions, crosslinking by irradiation raises and may also broaden the temperature range for heat sealing, and may also enhance the puncture resistance of the film.
Disadvantageously, the higher sealing range of crosslinked thermoplastic films causes food processors to heat seal their products at a higher temperature which may have a deleterious effect on their packaged product and which also results in higher energy costs. Also, if the heat sealing layer of the thermoplastic film is crosslinked too heavily, the highly crosslinked layer is more difficult to melt or fusion bond which makes it difficult to achieve strong seals, particularly by impulse sealing the bag mouths after filling with meat or poultry. All of the bag seals (including those made by both the bag manufacturers and the food processor and those made by whatever means including either or both hot bar seals and impulse seals) must maintain their integrity to preserve and protect the enclosed food product. There must be a strong continuous seal to prevent unwanted egress and ingress of gaseous, liquid or solid materials between the bag exterior and interior. This is particularly necessary when the food-containing package is made of heat shrinkable film and is to be immersed in hot water to shrink the film against the packaged food since such shrinkage increases the stress on these seals. Thus, there is a continuing need for monolayer and multilayer films which can be made into bags having strong seals especially when formed by hot bar sealing or impulse sealing. Such films should provide strong seals able to withstand a range of temperatures and also be able to produce such seals over a wide sealing temperature range without burn through.
Variations in sealing temperatures, times and pressure are known to exist not only from one brand and/or type of sealers to another but also between different sealing machines sold by the same manufacturer under the same brand identification. Such variations, which may be due to factors such as variation in the manufacturer's product or varying equipment settings or installation, increase the desirability for films which may be heat sealed to produce strong integral seals over a wide range of temperatures and therefore be usefully sealed on different sealing machines.
Another problem encountered during heat sealing is that of inadvertent folding. Normally, a heat seal is made by applying heat and pressure across two sheets or portions of film e.g. the two opposing sides of a flattened tube, however, occasionally the area to be sealed will be inadvertently folded to produce a section of film having four or six sheets or film portions which are pressed between the opposing sealer bars. In such situations it is desirable to be able to seal the film without burn through. A wider impulse heat sealing temperature range is indicative of a greater latitude in sealing through folds than a narrower range.
Copolymers of ethylene and vinyl esters such as vinyl acetate have previously been disclosed as useful materials in monolayer and multilayer thermoplastic films and are known for providing heat sealing properties.
For example, U.S. Pat. No. 3,365,520 (Foster et al) discloses use of blends of polymers such as EVA with ethylene/acrylic acid/sodium acrylate (EAA) interpolymers to form biaxially oriented films. Blends of EVAs having a melt index of 0.4 dg/min. with EAAs having a melt index of 1.6 are disclosed. Such blends are said to provide an improved ability to biaxially orient.
U.S. Pat. No. 3,707,590 (Wiggins et al) discloses an oriented multilayer film which may have a barrier layer of vinylidene chloride-vinyl chloride copolymer and a heat sealing layer of an ethylene-unsaturated ester type copolymer such as ethylene-vinyl acetate copolymer (EVA). Two or more layers may be coextruded and biaxially oriented in a tubular film orientation process and a three layer film of EVA/Saran/EVA is suggested.
Canadian Patent 982,923 (Lustig et al) discloses a biaxially stretched multilayer film of EVA/polyvinylidene chloride/EVA which is heat shrinkable and suitable for packaging primal meat cuts. The heat sealing layer utilizes an EVA (12 wt. % VA) having a melt index of 0.3. Blends of EVAs are also disclosed.
U.S. Pat. No. 3,817,821 (Gallini) discloses use of EVA blends in multilayer films useful as snack food packaging.
U.S. Pat. No. 4,082,877 (Shadle) discloses use of a blend of EVA elastomers each containing about 28 weight percent of vinyl acetate and having melt indices of 6 and 23 in a multilayer film.
U.S. Pat. No. 4,127,688 (Bieler et al) discloses EVA/Saran/EVA multilayer, heat shrinkable film where the heat sealing layer containing EVA has been irradiated to increase crosslinking.
U.S. Pat. No. 4,064,296 (Bornstein et al) discloses a heat shrinkable multilayer film having an oxygen barrier core layer of hydrolyzed ethylene-vinyl acetate (EVOH) and outer layers of EVA. Use of EVA blends of varying VA content in the heat sealing layer is also discussed.
U.S. Pat. No. 4,178,401 (Weinberg et al) discloses an oriented, heat shrinkable packaging film having a blended self-welding layer said to have superior seal strength and abuse resistance. Blends of EVAs with different melt flows are disclosed with e.g. a first EVA having a melt flow of less than 5.0 blended with a second EVA having a melt flow of at least 28. The film may also be crosslinked by irradiation.
U.S. Pat. No. 4,247,584 (Widiger et al) discloses heat sealable food bags made from multilayer films having a heat sealing layer comprising a blend of EVAs with 10 to 90 weight percent of the blend comprising a first EVA having 2-12% VA and a melt index of 0.2 to 10 dg/min. and 90 to 10 weight percent of the blend comprising a second EVA having 8-30% VA and a melt index of 0.2 to 5.
Past attempts at providing improved heat sealing in films, while making some progress, leave much to be desired. Variability in heat sealing equipment and process parameters continue to produce bags with weak seals which are subject to burn through, which fail to seal through folds, and which produce leaking bags having discontinuous seals. It would be highly desirable to have biaxially stretched, heat shrinkable films and bags whose heat sealing layer in particular and film construction in general allows greater flexibility and variability in heat sealing process parameters while producing strong, integral, continuous seals rapidly and with a lower failure rate relative to prior art films and bags.